The overall goal of this proposal is to define the regulatory roles of collagen V in the injury response in normal and classic Ehlers-Danlos syndrome (EDS) tendons. Collagen V is a quantitatively minor component of collagen fibrils, yet modulation of its expression has dramatic phenotypic effects, indicating critical regulatory roles. EDS is a heritable connective tissue disorder with generalized connective tissue fragility and the classic form is characterized by hyperextensible skin, joint laxity and instability, as well as abnormal wound healing. Classic EDS is defined by collagen V mutations with haploinsufficiency for COL5A1 present in ~67% of affected individuals. Collagen V also has been linked to: Achilles tendinopathy, ACL rupture, as well as injury and performance deficiencies. In addition, upregulation of collagen V is observed in the injury response in tendons and ligaments. Our established haploinsufficent collagen V EDS mouse model will be used for the proposed studies. However, this model has limitations due to chronic effects resulting from lack of collagen V throughout development and maturation that are then superimposed on the changes associated with the injury response. Therefore, this proposal incorporates our inducible collagen V-null and heterozygous models that allow for temporal targeting of the modulation of collagen V expression. For the first time, directly studying the critical regulatory roles of collagen V during specific phases of the injury response will be possible. Using the haploinsufficient collagen V EDS model, we have demonstrated reduced fibril number, abnormal fibril structure, decreased cross sectional area, and a reduction in the stiffness of the flexor digitorum longus tendon consistent with the hypermobile EDS phenotype. Our recent work using a conditional null model demonstrated severe structural alterations at the fibril, fiber and tendon level associated with severely compromised mechanical function, joint dislocations and gait abnormalities. The general hypothesis tested in this proposal is that the tendon response to injury is modulated by collagen V mediated mechanism(s). The aims are to: (Aim 1) Define the altered regulatory function(s) of collagen V in the abnormal tendon injury response in classic EDS~ (Aim 2) Elucidate the mechanistic regulatory role(s) of collagen V in the repair response to tendon injury in a normal matrix~ and (Aim 3) Compare the effect of modulation of collagen V expression on the injury response in normal and in diseased/abnormal tendons. Our innovative approach with both conventional and inducible mouse models will systematically analyze the regulatory roles of collagen V on repair of tendon injuries. This approach will be coupled with sophisticated and innovative measures of mechanical and organizational properties, together with compositional profiles, to derive a mechanistic understanding of the injury responses. These studies will define the specific temporal regulatory roles of collagen V in the re- establishment of tendon structure and function in response to injury. In addition, the data will provide a critical foundation for developing therapeutic interventions to enhance the abnormal wound healing in EDS.